UNHCR head Filippo Grandi says refugees in Bangladesh in grave danger, with monsoon season beginning in March.

Rohingya refugees are still not allowed to return to Myanmar, the United Nations high commissioner for refugees has told the UN Security Council.

According to Filippo Grandi, "conditions in Myanmar are not yet conducive" for the 668.000 Rohingya to return home.

The refugees fled to neighbouring Bangladesh after the Myanmar authorities launched a violent crackdown in northern Rakhine state last August.

"The causes of their flight have not been addressed, and we have yet to see substantive progress on addressing the exclusion and denial of rights that has deepened over the last decades, rooted in their lack of citizenship," he said.

Grandi also said the office of the UNHCR lacks access to Rakhine, where hundreds of villages have been burned down by the Myanmar military.

"Humanitarian access, as you have heard, remains extremely restricted. UNHCR has not had access to affected areas of the northern part of Rakhine state, beyond Maungdaw town, since August 2017, and our access in central Rakhine has also been curtailed," he said.

"UNHCR presence and access throughout the state are essential to monitor protection conditions, provide independent information to refugees, and accompany returns as and when they take place."

Grandi recognised the efforts put in by both the government and the people of Bangladesh to house the Rohingya refugees, but warned that conditions have to improve for the hundreds of thousands of refugees especially with monsoon season starting in March.

"We are now in a race against time as a major new emergency looms. We estimate that more than 100,000 refugees are living in areas prone to flooding or landslides. Tens of thousands of particularly vulnerable refugees need to be urgently relocated," Grandi said.

"Their lives are at grave risk."

After Grandi gave his recommendations to the Security Council, Nikki Haley, the US ambassador to the UN, commended the UN had so far failed in its response to the crisis in Myanmar.

Haley, for her part, criticised Myanmar's leader and Nobel Peace Prize laureate Aung San Suu Kyi for failing to halt the violence against the Rohingya.

"This council must hold the military responsible for its actions and pressure Aung San Suu Kyi to acknowledge the horrific acts taking place in her country," Haley said.

The journalists were arrested while investigating a story about mass graves in Rakhine.

"For the Myanmar government, their ambassador said that the country respects the freedom of the press. It says the journalists were arrested because they broke state secrecy laws," our correspondent said.

Nearly 690,000 Rohingya have fled Rakhine and crossed into southern Bangladesh since August, when attacks on security posts by rebels triggered a military crackdown that the UN has said may amount to genocide.

Myanmar's government has denied the allegations.

Since August, the number of refugees fleeing to Bangladesh has gone down, with up to 1,500 arriving in the last month, according to the UN.

Last month, Bangladesh announced it would delay the repatriation of hundreds of thousands of displaced Rohingya amid fears over their safety once they return.